1. Field of the Invention
The present invention relates to an apparatus compensating for dispersion of an optical fiber in an optical line when an optical signal generated by an optical transmitter is transmitted to an optical receiver via the optical line, and, more particularly, to an apparatus compensating for dispersion of an optical fiber in an optical line using a dispersion compensation optical filter.
2. Description of the Related Art
When an optical signal is transmitted via an optical line in a high-speed optical transmission network, dispersion of the optical signal occurs. Accordingly, the maximum distance that the optical signal can be transmitted without compensation at a repeater is limited, and errors occur. To prevent these errors, dispersion of the optical line must be compensated. In general, the dispersion of the optical line is compensated by using a dispersion compensation fiber.
In order to calculate the total dispersion of optical fiber cables installed in the field, the optical fiber cables are produced in a factory, wound around a drum, and the ends of the produced fiber cable are connected to measuring equipment to measure the total dispersion. However, after being installed in the field, the ends of the optical cable are isolated from each other by tens to hundreds of kilometers, so it is difficult to measure the dispersion value in the field. Thus, a dispersion value per km is calculated, the length of a dispersion compensation optical fiber is thus determined, and the determined length is applied to compensate for the dispersion. However, this method does not produce accurate dispersion compensation.
In the prior art, when the optical fiber cables were produced in a factory and the dispersion measured, a conventional single-mode fiber typically has had a dispersion of, at most, 17 ps/nm/km at a wavelength of 1.55 .mu.m. Here, the dispersion value is positive. The positive dispersion can be minimized to, at most, 2.7 ps/nm/km if using a dispersion shifted fiber designed to have a zero dispersion at a wavelength of 1.55 .mu.m. However, since the dispersion affects the maximum distance between repeaters in high-speed transmission, a dispersion compensation fiber having a negative dispersion is needed. In particular, the conventional single mode fiber has a large dispersion, so that a dispersion compensation fiber must be utilized. That is, when the conventional single-mode fiber having a dispersion of 17 ps/nm/km is installed and used in a 40 km length, its total dispersion is 680 ps/nm/km. Accordingly, in order to obtain a zero dispersion, a dispersion compensation fiber having a negative dispersion of 680 ps/nm/km is applied to the optical line. However, the dispersion compensation fiber is modularized into 20 km, 40 km, 60 km, 80 km, etc. lengths, and installed and used in a rack of optical transmission equipment. Therefore, if the length of an optical line does not match one of the modularized distances, the dispersion cannot be accurately compensated, since an exact dispersion compensation fiber is not used.
As described above, use of the dispersion compensation fiber may cause the following problems. First, it is difficult to completely eliminate dispersion by attempting to match a total positive dispersion of an optical fiber in an optical line with a negative dispersion of a dispersion compensation fiber. Second, since dispersion varies with the deterioration of an optical fiber in the optical line, the dispersion compensation fiber cannot continuously compensate for the varying dispersion. Third, since the dispersion compensation fiber is modularized and packaged to have a constant dispersion, when dispersion compensation is made for an actual optical line, it is difficult to compensate the dispersion with a standard length of the dispersion compensation fiber. Since an optical line is installed with arbitrary length, modularization and standardization of the dispersion compensation fiber are not easy.